linux-kernel - Re: [PATCH v3 7/7] vhost: feature to set the vring endianness

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Date:	Wed, 8 Apr 2015 09:36:38 +0200
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 7/7] vhost: feature to set the vring endianness

On Tue, 7 Apr 2015 17:01:31 +0200
Cornelia Huck <cornelia.huck@...ibm.com> wrote:

> On Tue, 07 Apr 2015 14:19:31 +0200
> Greg Kurz <gkurz@...ux.vnet.ibm.com> wrote:
> 
> > This patch brings cross-endian support to vhost when used to implement
> > legacy virtio devices. Since it is a relatively rare situation, the
> > feature availability is controlled by a kernel config option (not set
> > by default).
> > 
> > The ioctls introduced by this patch are for legacy only: virtio 1.0
> > devices are returned EPERM.
> > 
> > Signed-off-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>
> > ---
> >  drivers/vhost/Kconfig      |   10 ++++++++
> >  drivers/vhost/vhost.c      |   55 ++++++++++++++++++++++++++++++++++++++++++++
> >  drivers/vhost/vhost.h      |   17 +++++++++++++-
> >  include/uapi/linux/vhost.h |    5 ++++
> >  4 files changed, 86 insertions(+), 1 deletion(-)
> 
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  void __user *argp)
> > +{
> > +	struct vhost_vring_state s;
> > +
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		return -EPERM;
> > +
> > +	if (copy_from_user(&s, argp, sizeof(s)))
> > +		return -EFAULT;
> > +
> > +	vq->legacy_is_little_endian = !!s.num;
> > +	return 0;
> > +}
> > +
> > +static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  u32 idx,
> > +					  void __user *argp)
> > +{
> > +	struct vhost_vring_state s = {
> > +		.index = idx,
> > +		.num = vq->legacy_is_little_endian
> > +	};
> > +
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		return -EPERM;
> > +
> > +	if (copy_to_user(argp, &s, sizeof(s)))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}
> > +#else
> > +static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  void __user *argp)
> > +{
> > +	return 0;
> 
> I'm wondering whether this handler should return an error if the
> feature is not configured for the kernel? How can the userspace caller
> find out whether it has successfully prompted the kernel to handle the
> endianness correctly?
> 

Yes you're right. I think -ENOIOCTLCMD as suggested by Michael is a good
candidate.

Thanks.

> > +}
> > +
> > +static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  u32 idx,
> > +					  void __user *argp)
> > +{
> > +	return 0;
> > +}
> > +#endif /* CONFIG_VHOST_SET_ENDIAN_LEGACY */

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